The present invention relates to seat structures, and more particularly, seat structures of land motor vehicles capable of occupying a plurality of predetermined positions of adjustment.
The contemporary tendency of motor vehicle manufacturers is to design so-called "monospace" or "one-volume" vehicles which are akin to both the sedan called a station wagon and the utility vehicle called a van. In view of the intended universitality of utilization of these vehicles, they must be equipped with seat structures capable of taking up preferably predetermined, multiple and varied positions, i.e. a certain number of discrete positions, each of which is well defined. Seat structures of this type must for example be capable of being completely folded into a bed, an easy-chair or reclining chair, a normal driving seat, or partly folded into a small table or shelf; they also must be capable of being partly folded away or completely folded up in a compact manner to a relatively small size for the purpose of stowing away in the vehicle or removing them from the latter.
Such seat structures must also satisfy safety standards drawn up by professional bodies or public authorities so that the occupant of a seat structure is effectively retained thereon by a safety belt in event of violent shocks having in particular a large longitudinal component with respect to the vehicle.
This last-mentioned requirement becomes particularly difficult to satisfy when, as is becoming more and more common, such a safety belt is no longer directly anchored at two or three points of the frame or body of the vehicle, the safety belt being as it were independent of the seat structure, but is on the contrary integral with the seat structure and as it were carried by the latter, the different anchoring points for the safety belt being established directly on the seat structure itself. In such a situation in which the belt is carried by the seat structure, in the event of violent shocks, the forces to be absorbed and/or transmitted are consequently located essentially, on one hand, in the region of the axis of relative inclination between the backrest and the seat of the seat structure, and, on the other hand, in the region of the base mounting usually securing the seat of the seat structure to the floor of the vehicle.
It will be understood how difficult it is to satisfy all these prescriptions, which are often contradictory, especially when it is considered that such a seat structure must be mass-produced at the lowest possible cost.
A solution has been proposed in document EP 0 121 452 for a seat structure articulated at a single position which is adjusted in a substantially continuous manner for the case where such a seat structure is not equipped with a safety belt carried by the seat. The seat structure according to this document mainly comprises a seat equipped with a frame provided with a bracket, a backrest equipped with a frame with a bracket, an articulation which interconnects said brackets, permits the backrest to swing relative to the seat so as to change its inclination, and is constituted by a bearing and a pin engaged in the latter, a locking mechanism which normally immobilizes the backrest relative to the seat in the position in which it has been placed, permits changing the position at will, and is constituted by a toothed sector fixed to one of said brackets and centered on the pin of the articulation, a toothed block adapted to cooperate with said toothed sector and movably mounted on the other of said brackets by means of at least one guide carried by the latter so as to be movable thereon, and a hand control which comprises a movable cam cooperating with a counter-cam fixed to the toothed block and an elastically yieldable biasing device which normally maintains the cam and counter-cam, and the toothed block and toothed sector applied against one another so as to establish and maintain a contact, in particular between the toothed block and the guide.
This solution is satisfactory, since it permits the construction of a seat structure without any operational play, whether this results from manufacturing tolerances or wear in the course of time. However, such a solution is unsuitable as such in the situation where a seat structure is equipped with a safety belt carried by the seat structure and must be capable of absorbing and/or transmitting forces to which the seat structure is subjected in the event of violent shocks having a large longitudinal component which comes from both the front and or the rear, and when such a seat structure is of a type having multiple discrete positions and occupies its "normal" driving position or a reclining or relaxing position close to the driving position.
Indeed, in the last-mentioned situation, an occupied seat structure must be capable of supporting and resisting without damage accelerations and decelerations exceeding ten g, and in order to achieve this with the solution of this document it is necessary to give the toothed block and the toothed sector great thickness and to employ a tooth having a large modulus. These considerations result in heavy and space-consuming structures with a relatively coarse adjustment, which features are very poorly suited to a practical utilization in mass production.